1. Field of the Invention
The invention relates to continuous casting of preliminary steel sections, such as, for example, preliminary I-sections.
2. Description of Related Art
Preliminary steel sections represent primary material for producing rolled sectional steel beams of I, H, U and Z cross-sectional shape as well as special sheet pile sections. A method for the continuous casting of preliminary sections of this kind is disclosed, for example, in EP-B-1 419 021. The continuous casting of preliminary sections was introduced on an industrial scale in the seventies and has been increasingly gaining in importance in recent years in consequence of the general trend towards so-called near net shape casting.
The preliminary sections are in most cases cast in an I-cross-sectional shape, the molten steel being introduced substantially vertically into a so-called “dog-bone” continuous mold whose mold cavity cross-section is composed of two flange parts and a web part. A preliminary sectional strand with a molten core is fed from the mold to a strand guide with secondary cooling devices.
Unlike the continuous casting of conventional long products of a rectangular or round cross section, the continuous casting of preliminary I-sections represents several problems, in particular in the case of preliminary sections with a relatively thin web part, when high strength special steel grades (CaSi or Al-killed and microalloyed steels with V, Nb, inter alia) are cast, or in the case of high-speed casting. For reasons of space, although also governed by economics, the molten steel is only introduced into the mold via one ingate, in most cases asymmetrically at the transition between the web part and one of the flange parts. It is consequently particularly difficult to fill the complicated mold cavity uniformly and without disturbing turbulence and thus create favorable conditions for the initial solidification while preventing near-surface casting defects (gas bubbles, pin holes). It is also difficult to obtain a symmetrical liquid flow inside the strand shell and consequently a symmetrical temperature distribution, which ultimately results in a homogeneous solidification structure. It is equally problematic, where a thin web part is concerned, to prevent arching during solidification and resultant core porosity and/or shrink holes.
A continuous mold for the continuous casting of preliminary I-sectional strands is known from JP 08 294746 A. Molten steel is introduced into the two flange parts via 2 submerged nozzles. In order to prevent surface defects on the preliminary sectional strand, it is proposed that a pair of static magnetic poles with S or N poles be disposed outside of the mold cavity both on the two flange outer sides and on both sides of the web part. Through the static magnetic field just below the mouth of the two submerged nozzles, the steel jet emerging from the submerged nozzles is to be slowed down and flow back in a horizontal flow to the mold wall and along this to the liquid surface. The static magnetic fields with N and S poles gives rise to a slowing-down effect of the vertical discharge flow from the submerged nozzles and an uncontrolled deflection from the vertical flow. This prior art does not refer to controlled, reversible traveling fields or flows in the molten crater for creating controlled flow and temperature conditions in the crater of the preliminary sectional strand.